Quite simply to have a quick and dirty same day offering for base level support without having to worry about nouveau eventually trying to provide features that they may not want to have in a opensource solution. So they can either maintain the status quo or submit patches that may drastically alter nouveau's offering. Right now their cost and resources spent in the opensource solution is minimal.
They'd better improve it enough to make it worthwhile...nv does not support the newest cards at the moment.
If nouveau does reach the level where it's feature complete then I'd say give nv the ol' heave ho